This invention relates to a vinylidene fluoride elastomer composition containing a specific iminium compound as a curing accelerator.
Vinylidene fluoride cured elastomers (fluoro rubber) have been used for various purposes such as O-rings, gaskets, diaphragms, hoses, sheets, rools and the like, and the demand for these elastomers has recently increased more and more.
There have been proposed many kinds of vinylidene fluoride elastomer compositions containing a polyhydroxy aromatic compound as a curing or cross-linking agent to improve compression set and scorch resistance properties. The improvement of these properties has been studied mainly from the view point of development of a curing accelerator to be used together with the polyhydroxy aromatic compound being a curing agent. For example, U.S. Pat. Nos. 3,655,727 and 3,876,654 teach the use of a quaternary phosphonium compound as a curing accelerator. U.S. Pat. No. 3,655,727 also teaches the quaternary ammonium compound. The specific quaternary ammonium compound of 8-alkyl- or 8-aralkyl-1,8-diaza-bicyclo[5,4,0]-7-undecenium and the amino phosphinic derivatives are disclosed in U.S. Pat. Nos. 3,857,807 and 4,259,463, respectively. In addition, the Bis(triarylphosphin)iminum compound was recently reported in European Patent Application No. 841031 26.3 (EPO 0120462 AI).
However, the values of compression set, especially at high temperatures, of the rubbers obtained from these fluoroelastomer compositions do not reach adequately satisfactory levels. Further, when the rate of curing is retarded by a reduction in the amount of the curing accelerator to prepare complicated or thick-gauge articles, adhesion of the cured elastomer to a mold occurs which induces the appearance of defects in the articles and contamination of the mold.
In the case of a composition employing the quaternary ammonium compound, in addition to the above problems, there are some other problems, which are induced by hygroscopic and deliquescent properties. These problems include the necessity of the troublesome consideration for safety storage of the composition and a decrease in the curing property as well as various properties of the obtained cured elastomer.
In the case of the Bis(triarylphosphin)iminum compound, as shown in the EXAMPLES hereinafter, the cured elastomer obtained shows a higher compression set, especially at a higher temperature, low elongation and large variations of strain property in the thermal aging test.